1. Field of the Invention
The present invention relates to an electronic device such as a timepiece device including a stepping motor and to a method of controlling such an electronic device. More particularly, the present invention relates to an electronic device capable of driving a stepping motor at a higher rate than in a normal mode and to a method of controlling such an electronic device.
2. Description of the Related Art
A stepping motor, also called a pulse motor, a stepper motor, or a digital motor, is widely used as an actuator in a digital controlling apparatus wherein the stepping motor is driven by a pulse signal. In recent years, various electronic devices have been developed which are small in size, suitable for a user to carry. In such electronic devices, a small-sized light-weight stepping motor is widely used as an actuator. A representative example of such an electronic device is a timepiece device such as an electronic clock, a timer switch, chronograph, etc.
FIG. 13 illustrates a typical structure of a stepping motor 10 used in a timepiece device. As shown in FIG. 13, the stepping motor 10 includes a disk-shaped rotor 13 magnetized in a two-pole form. The rotor 13 is adapted to rotate in a stator 12 including parts combined together into the form of a single piece with a magnetic saturating part 17 having a notch formed on the outer end face. As the rotor 13 rotates in response to driving pulses supplied at a proper frequency such as 1 Hz, hands of the timepiece device, which are geared to the rotor 13, are driven by a driving force of the rotor 13. To correctly drive the hands without producing an error, it is important to monitor whether the rotor 13 is successfully and properly driven to rotate. To this end, a current or voltage is detected that is induced in the driving coil by a back electromotive force created by the rotation of the rotor 13.
In the current induced by the back electromotive force generated by the rotation of the rotor 13, as shown in FIG. 14, a first peak PM1, with a polarity opposite to that of the driving pulse DP, appears when the rotor 13 passes through the position different in angle by approximately 90.degree. from the stable position. As the rotor 13 further rotates, the current induced by the electromotive force becomes zero at the position A. After that, a first large peak PP1, with the same polarity as the driving pulse DP, appears in the induced current when the rotor 13 passes through the stable position B, which is the destination (in the opposite pole) and which is different in angle by 180.degree. from the starting position. After that, second peaks PM2 and PP2 appear in response to a vibration around the stable position B until the rotor 13 finally comes to rest at the position B.
Although the first peaks PM1 and PP1 have large values, a transient current TW, due to the driving pulse DP, makes it difficult to distinguish in time the driving pulse DP from the first peak PM1 or PP1. For this reason, in the conventional technique, the second peak PM2 or PP2, which can be easily distinguished from the driving pulse DP, is employed to detect the rotation in the normal hand driving mode. To amplify the peak level, a chopper-assisted amplification technique is used.
In recent years, timepiece devices, such as wristwatches, have been formed with various capabilities. An example is the ability to drive the stepping motor more quickly than in the normal hand driving mode so as to automatically or manually adjust the time. To drive the stepping motor in the quick driving mode, it is required to supply quick driving pulses at intervals shorter than in the normal driving mode. Furthermore, during the quick driving operation for time adjustment, it is required to correctly drive the hands, that is, it is required to drive the rotor with no error. To meet the above requirements, it is required to determine, as early as possible after delivery of the driving pulse DP, whether the rotor 13 has been successfully driven. From this point of view, it is desirable to detect a first peak PM1 or PP1, or a phenomenon associated with the first peak PM1 or PP1 such as a transition of polarity of the current induced by the back electromotive force. To this end, it has been proposed to provide a detecting coil wound around a stator 12 separately from the driving coil so that the current induced by the back electromagnetic force is detected by the detecting coil. If the detecting coil is electrically isolated from the driving coil, the influence of the transient current caused by the driving pulse is suppressed, and thus it becomes easier to detect the first peak PM1 or PP1 or a phenomenon associated with the first peak PM1 or PP1.
However, the current induced by the back electromotive force flowing, as shown in FIG. 14, through the detecting coil creates a magnetic force in a direction against the rotation of the rotor 13. That is, the detecting coil acts as an electromagnetic brake against the motion of the rotor 13. Therefore, it is required to increase the effective power of the driving pulse so as to create a large enough driving force taking the braking force into account. This results in an increase in electric power consumed to drive the stepping motor. In particular, in the quick driving mode, a greater backward current is induced in the detecting coil because of higher-speed rotation of the rotor 13, and thus the detecting coil creates an even greater braking force. To drive the rotor at a required high speed against the large braking force, it is needed to supply driving pulses with a greater effective power. As a result, the power consumption further increases. Furthermore, the fact that the braking force increases with the speed of rotation makes it difficult to drive the rotor at a high speed.
Also, in the operation of detecting the rotation of the rotor 13 at an early juncture, where there is a transient current or where there is a timing error, there is a possibility that the rotation of the rotor 13 cannot be detected and thus the detection timing is not obtained. In this case, to continue the quick driving operation, it is required to supply a quick driving pulse with an opposite polarity with proper timing. However, unlike the normal hand driving operation in which driving pulses are supplied with fixed timing, for example at 1 Hz, it is impossible to correctly determine the timing corresponding to the rotation of the rotor 13 being quickly driven. Therefore, if the position of the rotor 13 fails to be detected, it becomes impossible to supply a quick driving pulse with proper timing. Thus, the driving pulse supplied with wrong timing can cause a braking force against the rotation of the rotor 13. Furthermore, a first timing error in supplying a quick driving pulse can cause timing errors one after another in the following operation. Thus, once a timing error in supplying a driving pulse occurs, the following timing tends to become incorrect, and thus it becomes difficult to continue the quick driving operation in a stable fashion.
In view of the above, it is an object of the present invention to provide an electronic device that includes a stepping motor, and a method of controlling such an electronic device, which uses a detecting coil and is capable of determining whether or not the rotor of the stepping motor has been successfully driven without creating a significant braking force due to a current induced in the detecting coil.
It is another object of the present invention to provide an electronic device capable of quickly driving the stepping motor in a stable fashion with less power consumption than devices found in the prior art.
It is yet another object of the present invention to provide an electronic device and a method of controlling an electronic device, in which, when the position of the rotor is not successfully detected during the operation of driving the rotor by outputting driving pulses in accordance with the angular position of the rotor, a quick driving pulse is supplied with timing corresponding to the rotation of the rotor thereby ensuring that the stepping motor can be quickly driven in a highly reliable fashion.